1. Field of the Invention
The present invention relates to a photoelectric conversion device and an electronic appliance.
2. Description of the Related Art
A large number of photoelectric conversion devices used for detecting electromagnetic waves are known and, for example, photoelectric conversion devices having sensitivity to UV rays to infrared rays are collectively referred to as optical sensors. Among them, an optical sensor having sensitivity to visible rays with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices and the like which need luminance adjustment, on/off control, or the like in accordance with a human living environment.
In some display devices, ambient brightness of the display devices is detected so as to adjust the display luminance. This is because the visibility can be improved and surplus power consumption of the display device can be reduced by detecting the ambient brightness by an optical sensor and providing appropriate display luminance. For example, examples of a display device which has an optical sensor for adjusting the luminance include cellular phones and computers provided with display portions. In addition, as well as the ambient brightness of the display portion, the luminance of the backlight of a display device, a liquid crystal display device in particular, is detected by an optical sensor so as to adjust the luminance of a display screen.
An optical sensor, which is a photoelectric conversion device, includes a photoelectric conversion element such as a photodiode in a light sensing portion. Based on the amount of current flowing to the photoelectric conversion element, the light intensity can be detected. Patent Document 1 describes an optical sensor of charge accumulation type, specifically describes a structure in which electric charge accumulated in a capacitor (also called a capacitor element) by current flowing from a photodiode in accordance with the amount of incident light is discharged by a constant current circuit (also called a constant current source) and then the change in potential due to the discharge is detected by a comparator and based on the time required for the change in potential which is detected by the comparator, a digital signal is generated with use of a counter circuit and a latch circuit.